1. Field of the Invention
This invention relates to the field of aerodynamics, and in particularly to a method of controlling leading-edge vortex breakdown in high performance air vehicle.
2. Description of Related Art
The new generation of high-performance combat aircraft operates at high incidence and high angular rates. Under these flight conditions the airflow over the control surfaces is dominated by leading-edge vortices, which are exploited to obtain extra aerodynamic forces that help expand the performance envelope and thus gain tactical advantage in combat.
However, leading-edge vortex breakdown causes severe airload non-linearities and time dependence as well as highly unsteady flow downstream of the breakdown point. Control of leading-edge vortex behavior is of vital importance for enhancing maneuverability, which is reflected by the considerable research on potential active and passive control methods currently under way. By modifying vortex behavior, it is in principle possible to replace or complement conventional control surfaces, with their inherent limitations in this flight regime, with innovative active flow control concepts.
A number of different methods of flow control have been and are being investigated. For the purpose of controlling leading-edge vortex, only limited success has been achieved so far due to the unrealistic power requirement, low efficiency, complex structure and control method, etc.
Any method employed should meet the requirement of real applications, where the extra power required or engine bleed air must be minimized, yet the control methods must be effective in significantly altering the flow. The transfer function must be well behaved so that it can be incorporated into control laws and be simple and reasonably priced.
There are many methods aiming at controlling leading-edge vortex. Each method can also be used to control different phenomena, such as forebody vortex, boundary layer separation, and all kinds of vortex, either on the wing or over the wing as well as wing tip vortex and trailing vortex.
While there are many know such methods, one known method is to perform blowing, suction, or periodic blowing and suction. For example, U.S. Pat. No. 6,138,955 discloses the control of wind tip or trailing vortices, and in particular, blowing near the core at fixed location for arbitrary angle of attack or vortex breakdown location. At a blowing coefficient Cxcexc=0.013 the vortex breakdown location is delayed by 45% wing chord. This is also discussed in the paper by Guillot, S., Gutmark, E. J. and Garrison, T. J., xe2x80x9cDelay of Vortex Breakdown over a Delta Wing via Near-Core Blowing,xe2x80x9d AIAA Paper 98-0315. Such methods involve blowing at a fixed location. They require huge blowing rates and unrealistic power requirements to result in significant delays in vortex breakdown.
According to the present invention there is provided a method of controlling flow field on aerodynamic surface, comprising blowing a fluid jet into, or sucking a fluid jet out of, a fluid flow adjacent said surface; continually identifying a sensitive spot in said fluid flow field near a vortex breakdown region, and dynamically displacing a source of said fluid jet toward said sensitive spot to enhance, and preferably maximize, the effectiveness of said fluid jet.
In another aspect the invention provides an arrangement for controlling an aerodynamic surface, comprising at least one pressure sensor for developing a signal identifying a non-linear region where vortex breakdown occurs; and a fluid jet generator for generating a fluid jet that can be blown into or sucked out of said non-linear region to displace said vortex breakdown to a target location, said fluid jet generator being continually displaceable along a path on said aerodynamic surface to direct said fluid jet toward a sensitive spot in said fluid flow field near the vortex breakdown region.
The sensitive spot is typically just upstream of the vortex breakdown location. The location of the vortex breakdown can be found with the aid of pressure sensors in a manner known per se in the art.
The source of the fluid jet is preferably a nozzle that moves along a defined path on the aerodynamic surface, typically a wing of an aircraft. Alternatively, the source can be a series of nozzles extending along the defined path. The nozzles are individually activated to displace the source of the fluid jet. A typical number of nozzles would be 7, with anywhere between 5 and 10 being optimum. The moveable source can be also provided by a system of pressure sensors and MEMS devices that act as actuators controlling a system of valves on the wing.
The invention employs a closed-loop method that is capable of keeping breakdown where desired. The control flow can be applied through the moving nozzle or sequentially through adjacent nozzles to force breakdown to almost any arbitrary position. Symmetric and differential shifting of the breakdown location can be used to control the normal force and rolling moment respectively with realistic control power. The location of the sensitive spot(s) can be found with pressure sensors.
The use of synthetic jets operating over a range of frequencies rather than constant blowing may also be used as a means to increase the proposed method""s effectiveness.
Leading-edge vortex breakdown depends on the balance of vorticity feeding rate from the leading edge boundary-layer separation and its downstream convection rate. At the location where the convection rate is less than the feeding rate, the vortex core will be forced to become tilted. The titled vortex core introduces more unfavorable pressure gradient. This is a positive feed back resulting in the amplification of adverse pressure gradients and eventual breakdown of the vortex.
The invention is based in part on the discovery of the non-linear process of vortex breakdown. The inventors have discovered that the control power required to strengthen or destroy the vortex is a nonlinear function of the distance between the jet and breakdown locations and the orientation of the jet within the breakdown region. The key issue leading to the realistic jet power lies not in injecting the jet near the vortex core but, most importantly, in taking advantage of the nonlinear function and in finding sensitive spots in the flow field where low level blowing causes the breakdown point to move significantly so as to satisfy the aforementioned requirements in realistic applications.
If blowing is too far upstream of the breakdown or away from the nonlinear region, for example, compared with the global field, this imported energy is too small to have any effect on the global field and too little to affect the location of the breakdown. On the other hand, if the blowing point is too far downstream of the breakdown, it will take time and much energy for the large-scale turbulence flow back to an organized vortex.
The aerodynamic surface may be a delta wing, optimized for air vehicle maneuvering and just slightly upstream of the vortex breakdown location.
By maintaining the blowing location and orientation in the non-linear region of vortex breakdown where sensitive spots occur, the efficiency obtainable with the inventive method can be as much as ten times higher than that achievable with the prior art using a fixed source and yet the technique is simple and easy to apply in a practical situation.
The invention is applicable applied to super-maneuverable combat vehicles or the like.
Nomenclature
In the present description the following terms are employed:
alet area of nozzle opening
b wing span
c root chord
cf feedback coefficient
cxcexc      b    ⁢          xe2x80x83        ⁢    l    ⁢          xe2x80x83        ⁢    o    ⁢          xe2x80x83        ⁢    w    ⁢          xe2x80x83        ⁢    i    ⁢          xe2x80x83        ⁢    n    ⁢          xe2x80x83        ⁢    g    ⁢          xe2x80x83        ⁢    c    ⁢          xe2x80x83        ⁢    o    ⁢          xe2x80x83        ⁢    e    ⁢          xe2x80x83        ⁢    f    ⁢          xe2x80x83        ⁢    f    ⁢          xe2x80x83        ⁢    i    ⁢          xe2x80x83        ⁢    c    ⁢          xe2x80x83        ⁢    i    ⁢          xe2x80x83        ⁢    e    ⁢          xe2x80x83        ⁢    n    ⁢          xe2x80x83        ⁢    t    =                    a        jet            s        ·                  (                              u            j                                U            ∞                          )            2      
eb blowing effectiveness
r radial distance
Re Reynolds number based on wing root chord
s wing planform area
U∞ free stream velocity
Uj jet velocity at the nozzle
Ux, axial component of velocity
Uxcex8 azimuthal component of velocity
Ur radial component of velocity
Xb chordwise location of blowing jet
xb non-dimensional blowing location       x    b    =            X      b        c  
XVB chordwise vortex breakdown location
xVB non-dimensional vortex breakdown location       x    VB    =            X      VB        c  
xcex1 angle of attack
xcex2 sideslip angle
xcfx84 convection time   τ  =      c          U      ∞      
xcfx89x axial component of vorticity
xcfx89xcex8 azimuthal component of vorticity